1. Field of the Invention
The present invention relates to a seatbelt device, and more particularly to a seatbelt device provided with an energy absorbing member that mitigates a load received by an occupant from a webbing during a vehicle collision.
2. Related Technology
A conventional seatbelt device for holding an occupant of a vehicle or the like in a seat includes an emergency locking mechanism driven by a sensor that reacts to sudden acceleration, a collision, or deceleration so as to physically lock unreeling of a webbing thereby restraining the occupant effectively and safely. The seatbelt device is further provided with an energy absorbing mechanism that performs energy absorption in order to lighten a load on a chest portion of the occupant by allowing the webbing to unreel by a predetermined amount when a collision force generated by the collision is large such that a load exerted on the webbing in response to inertial movement of the occupant reaches or exceeds a preset predetermined value.
In a known conventional seatbelt device (a two-stage control type seatbelt device) for effectively protecting an occupant from an impact force generated during a collision, a restraining force is controlled in two stages by setting an unreeling force of the webbing to be large for a predetermined time period in an initial stage of the collision, during which a comparatively powerful force acts on the occupant, and then reducing the unreeling force. With regard to the energy absorbing mechanism used in this type of seatbelt device, one end portion of a torsion bar disposed coaxially with a spindle for retracting the webbing is attached to the spindle, and the other end portion of the torsion bar is attached to a lock member of the emergency locking mechanism. When a load acting on the spindle in the webbing unreeling direction reaches or exceeds a predetermined value during an operation of the emergency locking mechanism, impact energy is absorbed by causing the torsion bar to twist. Furthermore, impact energy is absorbed during the initial stage of the collision by an additional energy absorbing mechanism that uses bending, twisting, and so on of a wire (see Patent Documents 1 to 4, for example).
As shown in FIG. 14, a seatbelt device 100 described in Patent Document 1 includes a first energy absorbing mechanism that absorbs impact energy by causing a torsion bar 101 disposed coaxially with a spindle 102 to twist during an emergency. In addition to the first energy absorbing mechanism, the seatbelt device 100 includes a second energy absorbing mechanism provided with a strip-form bending element 107 that is fixed at a tip end thereof to a circumferential direction groove 104 formed in the spindle 102, bent back by a radial direction groove 103, and accommodated in a bend route 106 provided in a locking member 105 to be capable of passing through the bend route 106. During an emergency, the second energy absorbing mechanism causes the bending element 107 to rotate together with the spindle 102 such that the bending element 107 passes through the bend path 106. Accordingly, the bending element 107 undergoes partial plastic deformation, and as a result, the impact energy is absorbed.
The restraining force is controlled in two stages by activating the first energy absorbing mechanism and the second energy absorbing mechanism simultaneously in the initial stage of the collision such that a comparatively large load is applied to the webbing and by applying a smaller, more stable load in a later stage of the collision generated by the first energy absorbing mechanism alone. In this seatbelt device, a play C is provided when the bending element 107 is engaged with the spindle 102 for a predetermined time after the second energy absorbing mechanism is activated in order to suppress an increase in webbing tension.
Similarly, seatbelt retractors described in Patent Documents 3 and 4 perform control such that impact energy is absorbed in two stages using a torsion bar serving as a primary load limiting member and a wire serving as a secondary load limiting member.
The above referenced patent documents are identified as Patent Document 1: Japanese Patent Application Publication No. 2003-19945; Patent Document 2: Japanese Patent Application Publication No. 2004-231019; Patent Document 3: Japanese Patent Application Publication No. 2006-27604; and Patent Document 4: Japanese Translation of PCT Application No. 2006-501105.
In this type of seatbelt device, the webbing tension increases rapidly from activation of an emergency locking means to a point at which the spindle starts to rotate, and an extremely large load is generated from the moment at which a gap between related components disappears. When the spindle begins to rotate thereafter, friction from the rotary part changes from static friction to kinetic friction, which has a small frictional coefficient μ. Accordingly, the webbing tension decreases rapidly and then stabilizes at a predetermined load (a load generated by twisting of the torsion bar). Hence, the load applied to the webbing tends to be unstable in the initial stage of an operation.
As shown by a dotted line in FIG. 5, in the seatbelt device 100 described in Patent Document 1, the webbing is unreeled comparatively easily immediately after activation of the emergency locking means using the space or play C in the second energy absorbing mechanism. Further, when static frictional resistance changes to kinetic frictional resistance such that the webbing tension decreases, the space or play C disappears, thereby suppressing a rapid reduction in the webbing tension. As a result, variation in the webbing tension is suppressed, and an occupant restraining performance is stabilized. However, the load generated at the moment when the gap or play disappears between the related components, those constituting the second energy absorbing mechanism, remains large, and therefore the load generated during the initial stage of an operation of the seatbelt device may not always be stabilized sufficiently, which requires further improvement.